In the eyeglass industry, it is desirable to provide lenses having different shapes and sizes to accommodate different sizes and shapes of eyeglass frames. Generally, eyeglass lenses start out as lens blanks having certain optical properties designed to correct one or more defects in a patient's vision. The blanks are usually circular and of substantially larger dimension, for example 70 mm in diameter, compared to the relatively smaller finished lenses assembled into eyeglass frames. Lens blanks are routinely subjected to edge processing in an effort to adapt them to a selected size and shape of eyeglass frames.
Edge processing can be achieved using any one of several conventional techniques. According to one such technique, eyeglass frame manufacturers provide lens patterns that fit within the respective lens mounts of those manufacturers' eyeglass frames. When a patient selects a particular style and size of frame, a lens blank which has been formed to correct that particular patient's vision defect is placed in an edging apparatus along with the pattern or patterns provided by the frame manufacturer. The edging apparatus then traces the pattern and removes material from the periphery of the lens blank in accordance with the pattern.
Conventional pattern-based techniques have certain drawbacks associated with them. For example, a relatively large number of patterns must be provided, storage space is required for such patterns, and difficulties arise when the patterns become misplaced or when they are misaligned during tracing. Typically, one or more patterns must be provided for each different shape and/or size of eyeglass frame. The amount of storage space required for such patterns increases as the choices in eyeglass frame sizes and shapes expand. Therefore, while such conventional techniques may be adequate for some applications, there remains a need for a more convenient arrangement.
In order to provide a more convenient arrangement, efforts were made to eliminate or reduce the need for patterns by providing a tracing apparatus capable of tracing the lens mounts of eyeglass frames. The results of such tracings (i.e. trace data) then were used to provide edging information for use in edging a lens blank.
A conventional tracer typically includes a clamp assembly for clamping the frames in a fixed position, and an engager having a projecting surface for tracing the groove of the frames. Trace data is generated according to the position of the engager. The edger processes the edge of the lens blank to create an edge profile according to the trace data. Therefore, accurately tracing the groove of the frames ensures a proper fit of the lens within the frame opening.
Most conventional tracers are generally effective when the lens mounts in the eyeglass frames are substantially planar. However, many tracers encounter problems when tracing frames having a “high wrap”. The term “high wrap” as used herein encompasses shapes that have a curvature greater than 6 diopters. Typically, though not necessarily, high wrap is provided so that the eyeglass frame more closely follows the contour of the wearer's face.
In particular, the engager that performs the tracing in many conventional tracers is generally biased in a radially outward direction to engage a groove in the lens opening or mount of the eyeglass frame. This radially outward biasing remains effective so long as the groove in the lens mount extends in the same radially outward direction. Eyeglass frames with high wrap, however, tend to have grooves in the high wrap region which extend obliquely (and which can even approach the perpendicular) with respect to the radially outward direction. As the engager enters the high wrap region and the groove in the frame gradually transitions to a more upwardly extending orientation, gravity acts to pull the engager down and out from the groove. The engager therefore tends to disengage the groove, rendering the trace data inaccurate.
As described more fully in U.S. Pat. No. 6,618,952, the disclosure of which is incorporated herein by reference, a tracer apparatus for tracing frames having a relatively high wrap has been developed. The tracer disclosed in the '952 patent includes an object engager which engages and traces the frames, and an actuator which moves the object engager into contact with and then along the frames. However, the tracer disclosed in the '952 patent maintains the frames in a fixed position during tracing. The tracer apparatus is therefore adapted to account for high wrap of the frame.